Catalytic cracking, and particularly fluid catalytic cracking (FCC), is routinely used to convert heavy hydrocarbon feedstocks to lighter products, such as gasoline and distillate range fractions. There is, however, an increasing need to enhance the yield of light olefins, especially propylene, in the product slate from catalytic cracking processes. Light olefins (C2-C4 olefins) are important feedstocks for the petrochemical industry. Propylene, for example, a light olefin hydrocarbon with three carbon atoms per molecule, is an important chemical for use in the production of other useful materials, such as polypropylene. Polypropylene is one of the most common plastics found in use today and has a wide variety of uses for both as a fabrication material and as a material for packaging.
To produce light olefins, the catalytic cracking of heavy hydrocarbon feedstocks, such as naphtha, is typically carried out by contacting a naphtha-containing feed with a catalyst composition usually comprised of one or more crystalline microporous molecular sieves to selectively convert the feed into an olefin-containing mixture. Although various naphtha catalytic cracking processes have been proposed in the past, many of the processes do not produce commercially important light olefins, e.g., propylene, with sufficient selectivity or yield. Also, the cracking processes can produce undesirable amounts of methane and aromatics as unwanted byproducts. In contrast, a practical and economic naphtha catalytic cracking process should selectively produce increased amounts of light olefins, e.g., propylene, while producing minimal amounts of methane and aromatics.
In FCC processes, a hydrocarbon feedstock is injected into the riser section of a FCC reactor, where the feedstock is cracked into lighter, more valuable products upon contacting hot catalyst circulated to the riser-reactor from a catalyst regenerator. A major breakthrough in FCC catalysts came in the early 1960s, with the introduction of molecular sieves or zeolites. These materials were incorporated into the matrix of amorphous and/or amorphous/kaolin materials constituting the FCC catalysts of that time. These new zeolitic catalysts, containing a crystalline aluminosilicate zeolite in an amorphous or amorphous/kaolin matrix of silica, alumina, silica-alumina, kaolin, clay or the like were at least 1,000-10,000 times more active for cracking hydrocarbons than the earlier amorphous or amorphous/kaolin containing silica-alumina catalysts. This introduction of zeolitic cracking catalysts revolutionized the fluid catalytic cracking process. New processes were developed to handle these high activities, such as riser cracking, shortened contact times, new regeneration processes, new improved zeolitic catalyst developments, and the like.
The zeolites typically used in FCC are crystalline aluminosilicates which have a uniform crystal structure characterized by a large number of regular small cavities interconnected by a large number of even smaller channels. It was discovered that, by virtue of this structure consisting of a network of interconnected uniformly sized cavities and channels, crystalline zeolites are able to accept, for absorption, molecules having sizes below a certain well defined value while rejecting molecules of larger sizes, and for this reason they have come to be known as “molecular sieves.” This characteristic structure also gives them catalytic properties, especially for certain types of hydrocarbon conversions.
In current commercial practice, most FCC cracking catalysts used throughout the world are made of a catalytically active component large-pore zeolite. Conventional large-pore molecular sieves include zeolite X; REX; zeolite Y; Ultrastable Y (USY); Rare Earth exchanged Y (REY); Rare Earth exchanged USY (REUSY); Dealuminated Y (DeAl Y); Ultrahydrophobic Y (UHPY); and/or dealuminated silicon-enriched zeolites, e.g., LZ-210. ZSM-20, zeolite L and naturally occurring zeolites such as faujasite, mordenite and the like have also been used.
In addition to large pore zeolites, the ZSM family of zeolites is well known and their preparation and properties have been extensively described in the catalytic cracking of hydrocarbons. For example, one type of the ZSM family of zeolites is that known as ZSM-5. The crystalline aluminosilicate zeolite known as ZSM-5 is particularly described in U.S. Pat. No. 3,702,886, the disclosure of which is incorporated herein by reference. ZSM-5 crystalline aluminosilicate is characterized by a silica-to-alumina mole ratio of greater than 5 and more precisely in the anhydrous state by the general formula:[0.9.+−.0.2M.2/nO:Al2O3:>5SiO2]wherein M having a valence n is selected from the group consisting of a mixture of alkali metal cations and organo ammonium cations, particularly a mixture of sodium and tetraalkyl ammonium cations, the alkyl groups of which preferably contain 2 to 5 carbon atoms. The term “anhydrous” as used in the above context means that molecular water is not included in the formula. In general, the mole ratio of SiO2 to Al2O3 for a ZSM-5 zeolite can vary widely. For example, ZSM-5 zeolites can be aluminum-free in which the ZSM-5 is formed from an alkali mixture of silica containing only impurities of aluminum. All zeolites characterized as ZSM-5, however, will have the characteristic X-ray diffraction pattern set forth in U.S. Pat. No. 3,702,886 regardless of the aluminum content of the zeolite.
Beta zeolite is another zeolite that can be used in the catalytic cracking of hydrocarbons. Beta zeolite is typically a silicon-rich large pore zeolite having a three-dimensional pore structure, and has both acid catalytic properties and structural selectivity due to its structural particularity, and further has very high thermostability (the failure temperature of the crystal lattice is higher than 1200° C.), hydrothermal stability and abrasion-resistant properties. Due to the unique structural features, thereof, the zeolite beta has good thermal and hydrothermal stability, acid resistance, anti-coking properties and catalytic activity in a series of catalytic reactions. It has thus developed rapidly for new catalytic processes in recent years.
The catalysts used in FCC processing have been tailored to maximize the performance in specific hydrocarbon conversion processes. For instance, the catalyst compositions used in hydrocarbon conversion processes have been made into multifunctional catalysts, e.g., a bifunctional catalyst or a trifunctional catalyst. A bifunctional catalyst comprises two separate catalysts, e.g., two zeolites having different compositions or structure types, which induce separate reactions. The reaction products can be separate or the two catalysts can be used together such that the reaction product of one catalyst is transported to and reacts on a catalyst site of the second catalyst. Also, since one of the benefits of using a zeolite catalyst is that the catalyst is shape selective and non-selective reactions on the surface of the zeolite are usually not desirable, zeolite catalysts used in hydrocarbon conversion processes have the capability of preventing or at least reducing unwanted reactions which may take place on the surface of the zeolite catalyst by selectively sieving molecules in the feedstream based on their size or shape. Thus, undesirable molecules present in the feedstream are prevented from entering the pores of the catalyst and reacting. In addition, the performance of a zeolite catalyst can sometimes be maximized if the catalyst selectively sieves desired molecules based on their size or shape in order to prevent the molecules from exiting the pores of the catalyst.
Hydrocarbon conversions using catalyst compositions containing two different zeolites have been used in the past. For example, in order to increase the octane number of the gasoline fraction, a catalyst composition containing a large pore molecular sieve, such as zeolite Y, as the primary cracking component and a medium pore zeolite, such as ZSM-5, added to the zeolite Y cracking catalyst is typically used in conventional processes for catalytic cracking of heavy hydrocarbon feedstocks to gasoline and distillate fractions. U.S. Pat. No. 3,758,403 discloses a catalyst using ZSM-5 zeolite and a large pore zeolite such as zeolite Y (with a ratio of 1:10 to 3:1) as active components. In addition to enhancing the octane number of the gasoline, this catalyst mixture provides a higher yield of C3 and C4 olefins.
It is also known, e.g. from U.S. Pat. No. 5,279,726 and EP 559,646, to form composites of two different aluminosilicates, a Y zeolite and zeolite beta, for use in hydrocarbon cracking. In U.S. Pat. No. 5,279,726 a hydrocracking catalyst is disclosed having high activity and selectivity for gasoline which comprises a hydrogenation component on a catalyst support comprising both zeolite beta and a Y zeolite. In addition, U.S. Pat. No. 5,536,687 involves a hydrocracking process using a catalyst containing crystals of zeolite beta and zeolite Y that are bound by an amorphous binder material such as alumina.
In addition, CN 1103105A and EP-2-075-068 A1 describe the use of catalyst compositions comprising three different zeolites in hydrocarbon cracking. CN 1103105A discloses a cracking catalyst capable of giving a higher yield of isobutene and isopentene than without the catalyst and can coproduce high octane level gasoline. The components and contents of the catalyst described in CN 1103105A are as follows: (1) 5-25 wt. % modified HZSM-5 with a silicon:aluminum ratio of 20-100; (2) 1-5 wt. % of high silicon HZSM with a silicon:aluminum ratio of 250-450; (3) 5-20 wt. % of USY zeolite; (4) 1-5 wt. % of beta zeolite; (5) 30-60 wt. % of natural clay; and (6) 15-30 wt. % of inorganic oxide. EP-2-075-068 A1 describes a catalyst composition with a zeolite mixture as follows: (1) 1-75 wt. % of a zeolite beta modified with phosphorus and a transition metal; (2) 25-99 wt. % of a zeolite having a MFI structure, such as ZSM-5; and (3) 0-74 wt. % of a large pore zeolite, such as a Y zeolite.
To increase the yields of light olefins during the hydrocarbon cracking process, a zeolite cracking catalyst with added phosphorus has been used. WO 98/41595 discloses that the addition of a phosphorus-containing, medium pore zeolite, such as ZSM-5, to a conventional large pore molecular sieve cracking catalyst increases the yield of C3 to C5 olefins in the catalytic cracking of hydrocarbon feedstocks without significant loss in the aging characteristics of the medium pore additive. Thus, the yield of C4 and C5 olefins in catalytic cracking can be enhanced by adding a phosphorus-containing medium pore zeolite, such as ZSM-5, to a conventional zeolite Y cracking catalyst.
Incorporation of the phosphorus in the medium pore zeolite is conveniently achieved by the methods described in U.S. Pat. Nos. 4,356,338, 5,110,776, and 5,231,064. Treatment with phosphorus-containing compounds can readily be accomplished by contacting the zeolite, either alone or in combination with a binder or matrix material, with a solution of an appropriate phosphorus compound, followed by drying and calcining to convert the phosphorus to its oxide form.
While the current FCC process of using various combinations of zeolite Y, beta zeolite, and ZSM-5 is an efficient process for converting heavier feed to lighter products, many times the process makes less than desirable amounts of light olefins like propylene. Growth in the polypropylene market is expected to drive the demand for propylene, and a production process of propylene via an FCC process that is more selective towards propylene than the prior art catalyst compositions is desired.